[en] This report describes the first phase of a programme of work that has as its overall objective the development of a computational model that can simulate the results of experiments on gas migration through highly compacted bentonite, and will provide the basis of a model suitable to assess the effects of bentonite barriers on the build-up of pressure and the escape of hydrogen gas from disposal canisters in a radioactive waste repository. In this first phase of the project, the possible mechanisms and controlling features of gas migration through compacted bentonite have been reviewed, and a preliminary computational model of the process has been implemented and evaluated. In the model it is assumed that gas invasion of the clay occurs by induced microfissuring, and that the permeability of the pathways thus created depends on the gas pressure (or the effective stress). Experimental data on gas migration in compacted bentonite that was collected under well controlled conditions by Horseman and Harrington was used in a preliminary evaluation of the new model. The model was able to reproduce qualitatively all the features seen in the subset of the experimental data used in the evaluation, and to provide quantitative agreement to substantial sections of the results of test sequences, but quantitative agreement between simulation and experimental results over a whole test sequence was not obtained. As part of the model evaluation, the dependence of the results obtained on key model parameters is reported. Outline plans for a further phase of work are suggested. (orig.)